tried to put together some 6AS7G equivalents with notes (from my personal experience). of course, the usual brand/re-label disclaimers apply. So no need which one is better. The goal here is just to list the main equivalents that may work in your amp's circuit. Did I miss any equivalents?

6080 - low gain @ 2- great vocals, wide soundstaging

7236 - medium gain - great bass, fast and dynamic. Almost SS sounding

5998A - medium gain in straight bottle. Plate construction like 6AS7.

5998 - medium gain at 5, full-bodied sound, same current load as 6AS7

421A - same as 5998 but with matched plates/sections

6AS7G - low gain, the standard tube for most, plentiful

6AS7GA - low gain, like the standard 6AS7G in striaght bottle

6520 - premium 6SA7G, sometimes with 5998 plates

6H13 - russian 6AS7G equivalent. nothing magical with these

ECC230 - european 6AS7G equivalent. haven't seen one in person

6528 - equiv to 5998 but very high gain at 9, twice the current load. Make sure you amp can handle it

6336 - like 6528 but low gain at 2, still twice current load. Make sure you amp can handle it

Oh, and I just talked with Mikhail today. He said he's developing a new adapter for the Extreme output socket so that it can handle a very different tube type -- I don't remember which tube it was though.

Oh, and I just talked with Mikhail today. He said he's developing a new adapter for the Extreme output socket so that it can handle a very different tube type -- I don't remember which tube it was though.

4X150A/ 7034/ CV2519 run VERY conservatively in the Extreme for 2.3 watts per channel.

Like you said at the top, this is what you experienced with you amp and input tube.

So as far as your description of sound though, that is very dependent on the input tube (and presumably the amp as well.)

For example with the 2c51 input, the 6AS7G's had a weird soundstage and sounded poor in my Extreme. But with the Tung Sol 5998's it was very natural and powerful.

But with some 12AT7's the 5998's sounded way too forward and bright, but were more natural with the 6AS7G's.

I think there is a 6080W also.
And the 2399.

The 6080W is about the same as the 6080 just a different manufacturing era .... there is the 6080WA /B /C.

The 2399 is a better quality 6AS7G with tighter section matching and apparently the ability to handle higher plate current. This tube in the pic has the plates that look like the 5998 plates .... which makes sense since most 5998s were made by TS and this example is a TS 2399. This tube is possibly almost identical to some of the 6AS7s with the 5998 plates. But, I think this tube sounds the same as all my other TS 6AS7s. Personally, unless you need more gain I like the 6AS7G as well as any tube in this series.

Personally, unless you need more gain I like the 6AS7G as well as any tube in this series.

I agree with this. I do think the Tung-Sol 6AS7G's sound a little better than the RCA's, and I think the GEC brown-base UK ones sound a little better still, but I don't think the differences are so huge that they are worth dealing with unless you are a psychotic tube collector...like me

Skylab - should you ever come across any NOS pairs of those I beg of you to alert me . I've been looking out for a few months now; there is one on eBay but only one (need a pair) and the seller hasn't got any others

There was a guy in the UK selling a whole bunch of these NOS for a while - but he must have sold out, since he doesn't seem to offer them now, although he has lots of other tubes for sale still. Yes, I will let you know if I see more of them for sale.

It's true. Only Tung-Sol made the 5998. You see them branded Chatham (which was bought by Tung-Sol), and IBM, and of course the most famous branding was the Western Electric 421a, but even these are 100% identical to the Tung-Sol 5998.

GE did make a "5998A", but this is a different tube. It has a plate structure more like a 6AS7 than the 5998, and is in a straight-bottle. They also sound nowhere near as good as the TS 5998.